A common problem with distributed temperature systems (DTS) (e.g., utilizing Raman DTS fibers) involves difficulties in making calibrated temperature measurements. Single-ended DTS measurements can only be calibrated with assumptions on the spectral-dependent loss characteristics of the optical fiber. Effects of hydrogen and stress on the fiber may change spectral-dependent losses in the fiber. Any changes to spectral-dependent loss require that temperature calibration points are included along the length of the sensing fiber, which can be particularly difficult to implement in certain applications (e.g., downhole configurations).
Furthermore, calibration points have very limited benefit if the spectral dependent loss is non-linear (location dependent). In the case of non-linear spectral attenuation, one accepted solution is to deploy a DTS sensing fiber in a dual-ended configuration, which, when interrogated from both directions by a DTS system on the surface, can be used to correct for errors due to non-linear spectral attenuation. To date, the primary technique to achieve a dual-ended DTS configuration has been to use two capillary lines (which may be, e.g., around ¼ inch in outer diameter) and a U-tube at the bottom that connects the two capillary lines, and then to pump an optical fiber along the entire length of the tubing (in the case of downhole applications, from the surface to bottom hole and back to the surface). However, this approach adds to the complexity of a completion (versus having only one capillary tube) and can also significantly increase the fluid pressures that are required to pump the DTS fiber/cable over the entire length of tubing.
What is needed is a DTS system with improved accuracy and without a U-tube configuration with dual capillary tube lines (such that installation costs and complexity of the system are reduced, along with reduced risk that the DTS fiber does not traverse the capillary lines).
Furthermore, where fiber Bragg grating (FBG) based distributed temperature sensing fibers are utilized, it must be recognized that such fibers are cross-sensitive to strain. While cabling techniques may be used to reduce strain imparted in FBG-based DTS fibers, completely removing strain and/or preventing changes in strain imparted to an optical fiber during and after deployment is difficult, if not impossible. What is also needed in the art is a FBG temperature sensing fiber having improved strain corrected temperature readings taken from the Bragg gratings.
Optical fiber temperature sensors, particularly those utilized in harsh environments, such as in downhole environments, are also predominantly plagued by temperature changes and drift sources. Thus, where measurement is attempted, additional sensors have been required to attempt to compensate for such temperature changes, and drift of the measurement. For example, two pressure sensors might be employed near each other having different sensor characteristics (i.e., different responses to the undesired parameter), and calculations may be made in an attempt to eliminate the effect of the parameter on the measurement (effectively in an attempt to isolate the parameter of interest, e.g., temperature effects at the point of interest).
While this may appear to be a good solution, conditions at the two sensors must be exact to accurately eliminate the influences of the undesired parameter. Also, the need to set up and run multiple sensors at every measurement point of interest can be tedious and costly.
What is also needed in the art is a simple, low cost solution to providing strain and drift source corrected temperature measurements in optical fiber sensors.